Magnetic recording media are widely used as audio tapes, video tapes, computer tapes, floppy disks, etc. A magnetic recording medium fundamentally has a structure in that a magnetic layer composed of ferromagnetic fine particles dispersed in a binder is formed on a non-magnetic support.
A magnetic recording medium is fundamentally required to be in high levels in various properties such as electromagnetic conversion characteristics, running durability, and running performance. In particular, with the recent increased propagation of 8 mm video tape recorders, etc., a video tape is required to have, in particular, excellent electromagnetic conversion characteristics such as a high video output and excellent original image reproducing faculty.
While there are various methods for improving the electromagnetic conversion characteristics of a magnetic recording medium, a method of improving the properties of ferromagnetic fine particles as a magnetic recording substance is a direct and effective method. Accordingly, the size of ferromagnetic particles becomes finer and finer for enabling high density recording and the material for ferromagnetic particles turns from iron oxide to iron oxide modified by a foreign metal such as cobalt, etc., and further recently, a ferromagnetic metal such as ferromagnetic iron, nickel, or cobalt or a ferromagnetic alloy of these metals has been used.
By using the ferromagnetic particles improved as described above, a magnetic recording medium having improved electromagnetic conversion characteristics can be theoretically obtained but, practically, it is difficult to produce a magnetic recording medium having improved electromagnetic conversion characteristics matching with the improvement of the ferromagnetic particles. The reason is as follows. That is, as the size of ferromagnetic particles is finer, the dispersibility of the ferromagnetic particles tends to reduce. As the property of ferromagnetic particles, there is the tendency that the dispersibility becomes lower and lower in the order of .gamma.-iron oxide, Co-doped .gamma.-iron oxide, and ferromagnetic metal (or alloy) fine particles. Thus, the improvement of ferromagnetic particles rather encounters a problem of reducing the dispersed state of the ferromagnetic particles in the magnetic layer, thereby the excellent characteristics of the ferromagnetic particles are not sufficiently obtained.
For improving the dispersibility of ferromagnetic fine particles, binders having a polar group such as -SO.sub.3 M, -OSO.sub.3 M, -PO.sub.3 M.sub.2, -OPO.sub.3 M.sub.2, or -COOM (wherein M represents a hydrogen atom, an alkali metal or ammonium) or binders having both the aforesaid polar group and an epoxy ring are disclosed, e.g., in JP-B-58-41565 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-57-44227, 59-30235, 60-238306, 60-238309, and 60-238371 (the term "JP-A" as used herein means as "unexamined published Japanese patent application"). These binders have a high absorptive power to ferromagnetic fine particles and show good dispersibility as compared to conventional binders.
However, even in the case of using such an improved binder, when the particle sizes of ferromagnetic particles become finer and finer for high density recording, the dispersion thereof become more difficult and hence the development of a technique of giving a binder having a faculty of highly dispersing such finer ferromagnetic particles has been desired.
For improving the dispersed state of ferromagnetic fine particles, there is a method of conducting kneading and dispersion of magnetic coating composition for a long period of time at the preparation of the coating composition but in this method, a considerable shearing force is applied to ferromagnetic fine particles at kneading and dispersing, thereby the properties of the ferromagnetic fine particles may be reduced, and further the long time required for the production of a magnetic recording medium gives a problem in production efficiency.
Also, a method of kneading magnetic composition using a specific binder such as a water-soluble carboxylic acid resin (JP-B-57-42888) or polyvinyl butyral (JP-A-59-165237) and then dispersing the mixture with the addition of other binder and solvent is proposed but a sufficient dispersion effect has not yet obtained.
Accordingly, a method of effectively dispersing the aforesaid ferromagnetic fine particles without largely changing conventional production processes of magnetic recording medium has been investigated and as such a method, there are known a method of using ferromagnetic particles subjected to a surface treatment with a surface treating agent such as a silane coupling agent, a method of using a component (dispersing agent) capable of improving the dispersibility of ferromagnetic particles, such as fatty acids, and a method of using a mixture of a vinyl copolymer and a urethane prepolymer added with a low molecular weight epoxy resin as disclosed in JP-B-56-23210.
However, in the aforesaid ferromagnetic particles subjected to the surface treatment using the silane coupling agent, the surface of the ferromagnetic particles is rendered hydrophobic by the silane coupling agent, thereby the compatibility of the ferromagnetic particles with resin components a binder is reduced although the stability in the dispersed state of the ferromagnetic particles in the magnetic coating composition is usually improved. Therefore, in the method, the final dispersed state of the ferromagnetic particles in the magnetic layer may not be sufficiently improved. Furthermore, since the silane coupling agent itself is very expensive, there is a problem in the use of such an expensive material for treating ferromagnetic particles for an ordinary magnetic recording medium.
A fatty acid, which is usually contained in a magnetic layer of a magnetic recording medium as a lubricant, also has a dispersing action for ferromagnetic particles. Thus, by controlling the amount of the fatty acid being used, the dispersed state of ferromagnetic particles can be improved but in general, when a fatty acid is used as a dispersing agent, a larger amount of the fatty acid must be used for obtaining the sufficient effect than the usual case of using the fatty acid as a lubricant for a magnetic layer. On the other hand, it is known that when a fatty acid is used in an excessive amount, it acts as a plasticizer for a binder. Therefore, when a fatty acid is used as a dispersing agent for ferromagnetic particles, there inevitably occurs a problem that the binder is plasticized by the fatty acid.
Apart from the above, there is a problem that by improving the dispersed state of ferromagnetic particles, the smoothness of the surface of the magnetic recording medium is increased to reduce the running property of the magnetic recording medium. For example, the dispersed state of ferromagnetic particles can be improved by using a large amount of a fatty acid but in this case, the magnetic recording medium is insufficient in durability and running property.
Also, when a low molecular weight epoxy resin is added to the specific binder as described above, the binder containing the epoxy resin is kneaded with ferromagnetic and the kneaded mixture is coated on a support. However, in this case, the kind of the binder being used is limited as well as the improvement of the dispersibility of ferromagnetic particles is insufficient, and if the kneading time is prolonged, the aforesaid problem that the characteristics of the ferromagnetic particles are reduced occurs.
It is known that a low molecular weight epoxy compound is incorporated in a binder as disclosed in JP-A-63-146218, and the epoxy compound is used as a component for a binder as in the case of JP-B-56-23210.
However, since in the magnetic recording medium described above, large amounts of binder and solvent are used for kneading and dispersing ferromagnetic particles, it is impossible to sufficiently strongly knead the composition.
Furthermore, if the particle sizes of ferromagnetic particles become very fine, the mechanical strength and the elasticity of the magnetic layer obtained by coating and drying the magnetic coating composition are increased but the shearing elongation of the magnetic layer is reduced and the magnetic layer becomes very brittle. Accordingly, powder dropping from the magnetic layer during VTR running is increased, and the troubles at the slitting step, i.e., the occurrence of cracks at the edge portions of the magnetic layer and the occurrence of powder dropping, are liable to occur.